The present invention relates in general to treatment of a selenium alloy for electrophotographic imaging members.
The formation and development of images on the imaging surfaces of electrophotographic imaging members by electrostatic means is well known. One of the most widely used process being xerography described, for example, in U.S. Pat. No. 2,297,691 to Chester Carlson. Numerous different types of photoreceptors can be used in electrophotographic imaging process. Such electrophotographic imaging members may include inorganic materials, organic materials, and mixtures thereof. Electrophotographic imaging members may comprise contiguous layers in which one or more of the layers performs a charge generation function and the other layer forms a charge carrier transport function or may comprise a single layer which performs both the generation and transport functions. These electrophotographic imaging members may be coated with a protective overcoating to improve wear. For Carlson type electrophotographic imaging processes, the protective overcoating must allow the electrostatic charge initially deposited on the outer surface of the overcoating to form at the interface between the protective overcoating and the underlying photoconductive layer prior to repeating the next imaging cycle. Protective overcoatings may be of various organic and inorganic materials including resins, photoconductive materials and the like.
Electrophotographic imaging members based on amorphous selenium have been modified to improve panchromatic response, increase speed and to improve color copyability. These devices are typically based on alloys of selenium with tellurium. The selenium electrophotographic imaging members may be fabricated as single layer devices comprising a selenium-tellurium alloy layer which performs both charge generation and charge transport functions. The selenium electrophotographic imaging members may also contain multiple layers such as, for example, a selenium alloy transport layer and a contiguous selenium-tellurium alloy generator layer.
A common technique for manufacturing photoreceptor plates involves vacuum deposition of a selenium-tellurium alloy to form an electrophotographic imaging layer a substrate. Tellurium is incorporated as an additive for the purpose of enhancing the spectral sensitivity of the photoconductor. Typically, the tellurium addition is incorporated as a thin selenium-tellurium alloy layer deposited over a selenium alloy base layer. Fractionation of the tellurium composition during evaporation results in a gradient in the deposited selenium-tellurium alloy layer during vacuum evaporation. A key element in the fabrication of tellurium doped photoreceptors is the control of fractionation of tellurium during the evaporation of the selenium-tellurium alloy layers. Tellurium fractionation control is particularly important because the local tellurium concentration at the extreme top surface of the structure, denoted as top surface tellurium, directly affects xerographic sensitivity and copy quality.
One method of preparing selenium tellurium alloys for evaporation is to grind selenium-tellurium alloy shot (beads) and compress the ground material into pellet agglomerates, typically 150-300 mg. in weight and having an average diameter of about 6 millimeters (6,000 micrometers). The pellets are evaporated from crucibles in a vacuum coater using a time/temperature crucible designed to minimize the fractionation of the alloy during evaporation. One shortcoming of the selenium-tellurium alloy layer in a photoreceptor structure is the propensity of the selenium-tellurium alloy at the surface of the layer to crystallize under thermal exposure in machine service. To retard premature crystallization and extend photoreceptor life, the addition of up to about 1 percent arsenic to the selenium-tellurium alloy was found beneficial without impairment of xerographic performance. It was found that the addition of arsenic to the composition employed to prepare the pellet impaired the capability of the process to control tellurium fractionation. Selenium-tellurium-arsenic pellets produced by the pelletizing process exhibited a wider variability of top surface tellurium concentrations compared to selenium-tellurium pellets. This wider variability of top surface tellurium concentrations was manifested by a correspondingly wider distribution of photoreceptor sensitivity values than selenium-tellurium alloy pellets. In an extended photoreceptor fabrication run, up to 50 percent of the selenium-tellurium-arsenic pellets were rejected for forming high top surface tellurium concentrations which caused excessive sensitivity in the final photoreceptor.